<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <div id="box1">
        <p id="para">我是段落</p>
    </div>

    <div id="box2">
        <p>我是box2的原有的p标签</p>
        <p>我是box2的原有的p标签</p>
        <p>我是box2的原有的p标签</p>
        <p>我是box2的原有的p标签</p>
    </div>
</body>

<script>
    var box1 = document.getElementById('box1');
    var box2 = document.getElementById('box2');
    var para = document.getElementById('para');
    var ps_inbox2 = box2.getElementsByTagName('p');

    // 移动方法1
    // box2.appendChild(para);

    // 移动方法2
    box2.insertBefore(para, ps_inbox2[0]);
</script>

</html>